Flowing salt water over graphene generates electricity

Dragging a drop around produces voltage.

Hydroelectricity is one of the oldest techniques for generating electrical power, with over 150 countries using it as a source for renewable energy. Hydroelectric generators only work efficiently at large scales, though—scales large enough to interrupt river flow and possibly harm local ecosystems. And getting this sort of generation down to where it can power small devices isn't realistic.

In recent years, scientists have investigated generating electrical power using nano-structures. In particular, they have looked at generating electricity when ionic fluids—a liquid with charged ions in it—are pushed through a system with a pressure gradient. However, the ability to harvest the generated electricity has been limited because it requires a pressure gradient to drive ionic fluid through a small tube. But scientists have now found that dragging small droplets of salt water on strips of graphene generates electricity without the need for pressure gradients.

In their study, published in Nature Nanotechnology, researchers from China grew a layer of graphene and placed a droplet of salt water on it. They then dragged the droplet across the graphene layer at different velocities and found that the process generated a small voltage difference.

In addition to being the first to demonstrate this effect, the scientists found a linear relationship between the velocity and the generated electricity. The faster they dragged the droplet across the graphene strip, the higher the voltage they generated. The scientists also found that the voltage increased when multiple droplets of the same size were used at once.

What’s the mechanism behind this? The scientists looked at the charge distribution on the sides of the droplet when it was sitting still on graphene, as well as when it was moving. When the droplet was static, the charge redistributed symmetrically on both sides, leaving a net potential difference of zero between them.

However, when the droplet was dragged across the graphene strip, this distribution became unbalanced. The scientists found that electrons are desorbed from the graphene at one end of the droplet and are adsorbed into the graphene at another end, which results in a large potential on one side of the droplet and generates a measurable voltage across its length.

The scientists then scaled this technology up to demonstrate that you can harvest electricity from it. They used a droplet made of copper chloride and placed it on a graphene surface. The surface was tilted to one side and the droplet was allowed to flow from one end to the other under gravity, resulting in the generation of a measurable voltage—approximately 30mV.

Although orders of magnitude lower than today’s hydroelectric generators, these nano-sized generators can work with small devices, something that hydroelectric systems can't do. And they can easily be scaled up, providing the potential to create large-scale generators.

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Ok, dumb question. How is allowing a drop to slide down a graphene sheet any different (mechanically) than relying on a pressure drop to push water through a capillary? In the end you're sacrificing a source of potential energy to move the fluid through the system. I'm guessing the relative energy must be lower since the pressure drop across small capillaries can be enormous.

The work here is interesting in that it probably won't work in a continuous flow application - there's no discontinuity in the surface potential to drive the voltage.

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Disclaimer: I am not a scientist. Obviously you need something to move the water across the graphene. Could that something be created by the movement of a person wearing a suit with water/graphene "patches" at certain positions? I'm thinking low-grade powered clothing here. Maybe not much, but enough to handle recharging duties on a "smart-suit" or similar.

Disclaimer: I am not a scientist. Obviously you need something to move the water across the graphene. Could that something be created by the movement of a person wearing a suit with water/graphene "patches" at certain positions? I'm thinking low-grade powered clothing here. Maybe not much, but enough to handle recharging duties on a "smart-suit" or similar.

Yes, this may be sci-fi rambling, but future-ho, thank you.

How about a dive suit that powers a hand-held propulsion system. They didn't mention anything about the current graphene could create, just the voltage, but it would be kind of cool if you had a battery powered under-water propulsion system that constantly recharged itself as you swim through the ocean. Kind of like an electric perpetual "motion" machine. Such a system probably wouldn't be 100% self sustaining, but it could definitely improve efficiency.

Ok, dumb question. How is allowing a drop to slide down a graphene sheet any different (mechanically) than relying on a pressure drop to push water through a capillary? In the end you're sacrificing a source of potential energy to move the fluid through the system. I'm guessing the relative energy must be lower since the pressure drop across small capillaries can be enormous.

The work here is interesting in that it probably won't work in a continuous flow application - there's no discontinuity in the surface potential to drive the voltage.

Disclaimer: I am not a scientist. Obviously you need something to move the water across the graphene. Could that something be created by the movement of a person wearing a suit with water/graphene "patches" at certain positions? I'm thinking low-grade powered clothing here. Maybe not much, but enough to handle recharging duties on a "smart-suit" or similar.

I was thinking of the old self-winding watches, but you're right, a suit or possibly something attached to your legs would probably doa better job. I guess it boils down to questions of scale, durability and longevity.

Interesting. My thoughts immediately went to shipping. What if large patches of a supertankers hull was coated this way? Could be a potential for "hybrid" tankers. Considering the amount of fuel oil they use, any fuel savings could add up to something not insubstantial.

Interesting. My thoughts immediately went to shipping. What if large patches of a supertankers hull was coated this way? Could be a potential for "hybrid" tankers. Considering the amount of fuel oil they use, any fuel savings could add up to something not insubstantial.

I'm almost certain the tanker would have to use more fuel to push it through the water.

This is akin to attaching a wind turbine to your car in order to get "free" energy.

Seems like this method requires a "drop" of liquid in order to form a charge differential as it slides across the graphene. I don't see the same approach working for a constant flow of liquid. So... how can this be scaled?

And while Volts are nice to have, potential without current isn't very useful. Is there any mention of power (or current) measured?

Interesting. My thoughts immediately went to shipping. What if large patches of a supertankers hull was coated this way? Could be a potential for "hybrid" tankers. Considering the amount of fuel oil they use, any fuel savings could add up to something not insubstantial.

I'm almost certain the tanker would have to use more fuel to push it through the water.

This is akin to attaching a wind turbine to your car in order to get "free" energy.

I'm assuming the idea is to use fuel or stored energy to get it started, and then the graphene surfaces to help it "coast" for a while. He didn't say anything about totally replacing fuel. But cutting usage even by 1/4 would be good.

I suddenly have mental images of a hydroelectric plant that uses salt water and giant graphene sheets instead of turbines.

Scalability will certainly be an issue for this-- it's hard just to make a small piece of graphene for small circuit devices, let a lone the massive amount that would be needed for large scale power generation. At this point, the the best way to make graphene is still the 'scotch tape method,' which as you can imagine isn't quite ready for industry level production.

This seems like it could be ideal for some sort of sensor, I can think of plenty of applications in which ions changing the voltage on a tiny device would be very very useful.

Disclaimer: I work in this field, and many of my colleagues have spent many hours bashing their heads against the wall trying to get the most minuscule pieces of suitable graphene for research projects

Interesting. My thoughts immediately went to shipping. What if large patches of a supertankers hull was coated this way? Could be a potential for "hybrid" tankers. Considering the amount of fuel oil they use, any fuel savings could add up to something not insubstantial.

Energy isn't free. Either this is breaking down chemical bonds or it's getting its energy from the movement. If it's chemical bonds, then coating a ship could work because then you're tapping into stored energy. But if this is getting its energy from the movement, then you're just converting energy.

If you're converting energy, then you'd be converting a portion of the movement of the ship into electricity, which means if you even had 100% conversion efficiency in both directions, you can not every do better than break even.

Seems like this method requires a "drop" of liquid in order to form a charge differential as it slides across the graphene. I don't see the same approach working for a constant flow of liquid. So... how can this be scaled?

I'm imagining a Rolex-like perpetual movement setup where all kinetic energy is routed into spinning a droplet of saltwater around in a circle on top of a disc of graphene and just harvest the electricity from that.

Interesting. My thoughts immediately went to shipping. What if large patches of a supertankers hull was coated this way? Could be a potential for "hybrid" tankers. Considering the amount of fuel oil they use, any fuel savings could add up to something not insubstantial.

I'm almost certain the tanker would have to use more fuel to push it through the water.

This is akin to attaching a wind turbine to your car in order to get "free" energy.

The waves do have "free" energy. It's not the flow of salted water that creates voltage, but the water/air interface at each side of the drop. So I guess just flowing water along the graphene surface doesn't create nor drag energy.On the other side, water projections and waves will, but probably not nearly enough to move the tanker.Maybe enough to provide emergency power for a radio and some light, though.

If you're converting energy, then you'd be converting a portion of the movement of the ship into electricity, which means if you even had 100% conversion efficiency in both directions, you can not every do better than break even.

Same here. It's not the ship's movements that provide the energy.

Edit:I'd appreciate explanation on the donwvotes, why would waves not be a valid source of energy? there are wave based power plants.

I'm imagining a Rolex-like perpetual movement setup where all kinetic energy is routed into spinning a droplet of saltwater around in a circle on top of a disc of graphene and just harvest the electricity from that.

Somewhere in europe, Steorn stirs itself back to life, sensing a new hype generation scheme...

A good numbers of electrical generators can be operated in reverse (usually very inefficiently due to differing design optimisations) as motors, and vice versa. I wonder if that is also the case here? MHD never gained any traction, but no-moving-parts propulsors and pumps have all sorts of benefits in longevity and operating noise.

It can be scaled up by using lots of sheets of graphene stacked up, and many droplets of water running side-by-side between them. Stick it in a river and it could charge a radio or a watch.

As I observed above, this technique will not work in bulk water. This only works when you have an air-water-air interface across the graphene along with the motion of said surface. Underwater all you're going to do is generate a tiny smidgeon of heat compliments of drag.

To all those wondering if this can be used to enhance propulsion, etc. please consider an energy balance. The electrical energy is coming at the expense of a force being exerted on a moving droplet (F * d = work).

They're using gravity because it's "free" in that if you've already lifted a droplet it will want to convert its potential energy (stored as mass at a height) into kinetic energy. The potential field is generated by sacrificing some of the potential/kinetic energy transfer.

So if you're using this to power a boat you have to push the boat through the water harder to balance the energy generated. This is not the same as regenerative braking in a car as you don't often want to stop a boat.

A good numbers of electrical generators can be operated in reverse (usually very inefficiently due to differing design optimisations) as motors, and vice versa. I wonder if that is also the case here? MHD never gained any traction, but no-moving-parts propulsors and pumps have all sorts of benefits in longevity and operating noise.

EEK! Don't mention those letters in that order! Fluids engineers the world over are running for cover.

So what if we hollow out wind turbines and put water and graphene in there. I have no idea if it would actually work, but it seems like it would generate more than a turbine alone.

There's no free lunch. You're slowing down the rotational energy of the blades to extract the electricity.

You'd have to do a balance to figure out which one is more efficient but I suspect the internal generator is going to win that one.

You could put this on the roof of a house and get power when it rains. However, there was no mention of the closed-loop current so all we know is there's a 30 mV potential. We have no idea what the conversion efficiency of the energy used to drag the water across the surface is compared to the electrical power that could be generated.

Edit: Also, there's not much in the way of ions in rainwater so this technique may very well not even work for that application.

edit: In regards to the lightning,I'm at work so I couldn't find a charging symbol.

I think this post requires some clarification. I have no idea what the energy density this technology will be capable of, but if it is able to be minaturized, I could see charging associated with movement say if you kept your phone in your pants and you walked around a bit. As long as a particular orientation isn't needed and you could get really small droplets up and down really small graphene lanes...

As I observed above, this technique will not work in bulk water. This only works when you have an air-water-air interface across the graphene along with the motion of said surface. Underwater all you're going to do is generate a tiny smidgeon of heat compliments of drag.

Idea: Layers of graphite-coated sheets (one side only) placed underwater and angled slightly downward with air drawn from the surface and released from the lower edges of the sheets. Lower side would be placed facing an ocean current. The current would force the water between the sheets, the air release would create a water-air interface, and perhaps we'd see some electrical current. No "free" energy as the motion comes from an existing current. There's the question of whether the air pump would use more electricity than generated, but the available generating capability is a big question mark anyway.